The following descriptions are provided to help the reader understand how the current inventor has proceeded to analyse circuits of which he is aware and to assist the reader in understanding the relevance of the circuit invention. However, these references are neither Intended to, nor do of themselves, provide any admission by the applicant that they are published as may be required for an assessment of novelty or obviousness or are common general knowledge according to the laws of and in any particular country in the world.
Several models of commercial switching amplifiers are known, including class-D amplifiers. Most utilise a system including a first order servo-loop amplifier whose output is connected to a modulation input of a pulse width modulator. An output of the pulse width modulator is connected to an input of an output switching stage. A negative feedback path connects an output of the output switching stage to an input of the servo-loop amplifier and an amplifier input is also connected to an input of the servo-loop amplifier. This system may be viewed conceptually as the output of the servo-loop amplifier, being an integral of an error signal, the error signal being proportional to the difference between the scaled output signal of the output switching stage and amplifier input signals. This integrated error signal is that which is fed to the said modulation input.
The pulse width modulator in some systems includes a triangular-wave oscillator, whose output signal acts as a carrier reference signal which is applied to an input of a comparator. In some systems that are less common, the carrier reference signal is a sawtooth waveform rather than a triangular waveform. An output of the servo-loop amplifier Is also applied to an input of the comparator: The comparator and the triangular wave oscillator act as the said pulse-width modulator, wherein an output of the comparator acts as the output of the pulse width modulator.
The servo-loop amplifier most often has a forward transfer function of which is a current-to-voltage integrator.
This system uses negative feedback to reduce distortion, that is, improve accuracy. However, this system is known to produce distortion intrinsically. That is, the system produces distortion even for near-perfect electronic components or, in other words, mathematically for idealised components.
In addition, electronic imperfections which are significant, for example in practical power output switching stages, produce further errors.
Details of a system utilising these basic functions just described is given in Motorola application note AN1042.
A simpler class-D amplifier, with no negative feedback or servo-loop amplifier and direct input signal modulation of the pulse width modulator, is utilised by a Zetex integrated circuit ZXCD1000. Assuming all components are ideal in such a system concept, this idealised system is known to produce no distortion in contrast to the servo-loop system described above. However, this direct modulation system in practice is known to have several problems compared to the servo-loop approach, namely:
The output noise is typically higher owing to no feedback.
The distortion resulting from the imperfections of less than ideal, practical, electronic components is greater at low frequencies, where negative feedback of the servo-loop system is of assistance.
The output signal of the direct modulation system is proportional to the output stage supply rails and is thus modulated by variations in these rails. Owing to negative feedback, this effect is reduced in the servo-loop system, particularly at lower frequencies for which there is more negative feedback.
Class-D amplifiers have been developed by Bang and Olufsen which that company calls its “ICEpower” products. The principles of this system are described in numerous Audio Engineering Society publications and patent U.S. Pat. No. 6,297,692. This discloses an analogue switching amplifier in which the overall amplifier dominant pole is set by elements both in the forward servo-loop amplifier paths and also in the negative feedback paths.
Some Bang and Olufsen ICEpower models 250A, 500A, 250ASP and 500ASP, that I was able to try illustrated, for me at least, the following performance results: For example, the distortion at 20 kHz at higher powers but below clipping, into 4 ohms did appear to me to be close to 1% (100 kHz measurement bandwidth). If the results were accurate, this is roughly 2 orders of magnitude worse than typical well designed traditional analogue amplifiers. From my general knowledge of this field, I suspect that the ICEpower units perform relatively well compared to some other brands of class-D amplifier products.
While I have referred to a specific class-D amplifier that is commercially available I am aware that units do vary and as such results should of themselves not be necessarily taken as confirmation, but they do suggest that there is some difficulty with such amplifiers.
A circuit utilising the ICEpower basic principles is published in “Radio Technique” December 2002, pages 58-64.
The same publication at pages 140-144 also discloses a common servo-loop system but instead of the servo-loop amplifier being a first order integrator, it is designed as a second order servo-loop system. This offers more feedback at low frequencies. Paradoxically, at higher frequencies, where little additional negative feedback is available relative to the first order system, the distortion is intrinsically worse owing to the shape of the signal at the output of the servo-loop amplifier.
An object of this invention is therefore to provide an amplifier improvement that assists in reducing distortion or at least provides the public with a useful alternative.